Well, legally, you don't have to tell the father about the pregnancy, but there are some complications you should consider when making this decision.
First, you may choose to leave the father off the birth certificate, but doing so means you will be forfeiting your right to attain child support payments. This exclusion also creates complications further down the track, such as attaining a passport and making major medical decisions affecting the child.
Second, if the father finds out and wishes to be involved with the child's upbringing, then he will be at liberty to pursue all avenues necessary to facilitate that involvement. This may result in prolonged and very costly court proceedings to attain parenting orders and possibly a declaration of parentage. Honestly, I would argue the personal cost of court will be far higher for you and your child than the cost of talking to your ex about parenting issues.
Third, under section 60B of the Family Law Act 1975, children also have a legal right to know, spend time and communicate with both parents on a regular basis, regardless of the status of relationship between said parents. If the father pursues involvement in the child's life, it will look very, very bad for you if you were found to have been violating that child's rights and failing to recognise the benefit to the child of having his/her father involved in his life.
Finally, I really implore you to think about how this will affect your child long-term. Studies have consistently shown that kids who grow up without knowing who their father is tend to be prone to mental illness, including depression and anxiety, and they are more inclined to engage in criminal or other offensive behaviours. They also experience problems in their own relationships, even with the present parent.
You may not want to speak to the father, but your child has a legal right to have a relationship with him, and not permitting the child to have that relationship may result in costly court proceedings, behavioural challenges in the child, and various other difficulties over the course of the child's lifetime. How you feel about the father is very insignificant compared to how the child is going to feel growing up without ever knowing him.
I encourage you to seek out counselling or a child expert to ensure you are fully aware of the consequences of this decision.